JP3503552B2 - Seamless pipe manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a seamless pipe capable of suppressing the occurrence of internal flaws even in a difficult-to-machine material without causing a biting failure.

[0002]

2. Description of the Related Art Generally, a seamless pipe is a hollow steel formed by mannesmann punching, press punching, or hot extrusion by using a billet on a round or square steel billet produced by processing a cast steel billet or by continuous casting. Made into a tube
After that, the product is drawn by a rolling machine such as an elongator, a plug mill, or a mandrel mill, and finally subjected to a process of being standardized by a sizer or a stretch reducer to obtain a product.

As a material for a seamless pipe, continuous casting into a round billet can be relatively easily performed, such as a general low carbon steel having a small alloy content, and a material having a good hot workability can be cast as it is. If there is poor hot workability, such as stainless steel, which is likely to cause porosity or segregation in the center of the billet as cast, a large work is applied to form a round billet.

The main reason for the deterioration of the workability is the segregation and porosity of the central part of the continuous slab. The porosity generation mechanism, which has a particularly large effect, generally causes voids in the final solidification stage of the slab, and the molten steel is essentially This is because when high alloy steel or the like is to be supplied, the molten steel is not supplied because the content of Cr and the like is high and the viscosity of the molten steel increases. For example, as for the Cr amount, as shown in FIG. 5 and FIG. 6, when the Cr content exceeds 0.5 mass%, the viscosity of the molten steel sharply increases, which easily causes porosity. In this way, when a material that may contain defects inside is used as it is, it undergoes a severe process called Mannesmann drilling at the beginning of the pipe manufacturing process, and the pipe inner surface defects due to porosity and segregation of the shaft core. Occurs. For this reason,
In particular, not only carbon steels such as steels containing a large amount of alloying components and free-cutting steels containing a relatively large amount of sulfur S that deteriorates workability, which are called difficult-to-process materials, but ordinary steel grades with high carbon content and Cr are added. It has been said that pre-perforation rolling is also necessary for the steel grades to improve the internal quality. For example, as a method for producing a seamless pipe material of high Cr steel, 400
A square-shaped bloom having a relatively large cross section of 520 mm is cast by a continuous casting machine or the like, and after heating, slab rolling or the like is used to change a round-shaped steel ingot having a small cross-section from a rectangular-shaped steel ingot, that is, a round billet. It's like manufacturing. In this way, the production of a seamless pipe material by adding pre-processing to a cast material leads to an increase in the number of processes and costs due to heating during pre-processing, compared to steel types that can be used as a raw material as cast,
There has been a demand for a method of producing a pipe without adding pre-processing as much as possible.

To solve this problem, many proposals have been made to improve the casting method and the tube manufacturing method.

Regarding the improvement of the casting method, a method of mechanically crimping a continuously cast material during casting and mechanically crimping defects is often applied to a slab or the like having a rectangular cross section. A major problem when applying this light reduction process to round billets is the deterioration of the slab shape caused by the reduction roll and the cracking of the solidification interface that can occur when the reduction is increased. If the round slab is simply pressed down by a pair of flat rolls, naturally the pressure lower part is crushed and the cross section is flattened. Moreover, due to the rolling reduction, tensile stress is generated in the cross-section in a direction orthogonal to the rolling direction, so that cracking is likely to occur. If the amount of reduction is increased in order to increase the porosity pressure bonding effect, the shape will move further from the true circle and the rate of cracking will increase.To use it as a round billet for seamless pipe manufacturing, roll the billet by rolling it. Serious problems such as the inability to carry the material and the unstable biting during drilling occur, and in the end, the internal quality of the slab remains inadequate. In order to solve this problem, a casting method using an elliptical mold has been proposed, for example, as disclosed in Japanese Patent Laid-Open No. 7-108358 (hereinafter, Prior Art 1), in which a portion to be pressed is made larger in advance. Although this method is effective under light pressure,
It is obvious that it causes a new defect due to the fluctuation of the molten metal surface and the drawing of the powder due to the non-uniformity of the flow at the time of casting as compared with the perfect circular mold, and it cannot be a complete solution.

On the other hand, also in the pipe manufacturing process, there are many cases in which a technique for preventing the generation of internal flaws due to the Mannesmann crack at the time of drilling was examined. For example, Japanese Patent Application Laid-Open No. 55-106611 (hereinafter referred to as Prior Art 2) proposes that not only the limit of occurrence of inner surface flaws but also the condition of punching can be defined not only for biting or uneven thickness. However, as described in Prior Art 2, the limit of occurrence of inner surface flaw depends on the material, temperature, and manufacturing method of the raw material, so that the continuous casting material and the so-called difficult-to-process material are Even if the technology is applied, the occurrence of defects cannot be prevented.

Further, there is a technique for suppressing the occurrence of inner surface defects by changing the inclination angle of the piercing roll from before biting to immediately after that, and immediately before and after the trailing edge slip and during steady rolling. Japanese Patent No. 1811008 (hereinafter, Prior Art 3). This prior art 3 applies a rational method in terms of suppressing the occurrence of internal defects. That is, as a means for suppressing Mannesmann cracking, it is desired to continue drilling in a state where the roll inclination angle is high. However, in that case, defective biting is likely to occur, so the angle of inclination is reduced only when biting or slipping out. That is. However, this technology is not universal. According to this technology, when the material is poor, it must be bitten at a fairly low angle of inclination, and immediately after that, the roll must be placed at a high angle of inclination to maintain efficiency. Load is generated. Therefore, there is a risk of failure, and it cannot necessarily contribute to the original purpose of improving the production efficiency. Moreover, since the flaw itself occurs during the biting at a low inclination angle, it is not possible to suppress the flaws at the time of biting and slipping out, that is, at the front and rear ends of the pipe. However, it is difficult to produce products with good inner quality while maintaining production efficiency. In addition, since it is necessary to design it so as to withstand a large mechanical load, the equipment cost becomes expensive.

On the other hand, when the workability of the material is poor,
There are many examples of examining the manufacturing method. For example, Patent No. 1
In No. 828639 (hereinafter, Prior Art 4), when manufacturing free-cutting steel seamless pipes such as sulfur free-cutting steel and lead free-cutting steel by the Mannesmann pipe manufacturing method, the outer diameter of the material tip is set to a roll opening or less. A technique for suppressing inner surface defects by appropriately tapering is disclosed. This prior art 4 does not exceed the frame in that the prior art such as Japanese Patent Laid-Open No. 56-89307 is cited and the material is processed, and the manufacturing cost cannot be reduced.

Further, in order to reduce the manufacturing cost, a technique for producing a pipe while pre-processing the raw material as little as possible is disclosed in, for example, Japanese Patent Laid-Open No. 1-282803, in which a specific steel type, here, Japanese Patent Application Laid-Open No. 6-106209, which is a prior art for limiting the working degree and heating temperature of a round billet continuously cast on a duplex stainless steel.
There is also a prior art which discloses a piercing and rolling method based on a geometrical relationship of a piercing machine and a plug as in Japanese Patent Publication No. The latter does not limit the material and seems to be a fairly effective proposal at first glance. However, if the material quality is poor,
As described in "Iron and Steel" magazine No. 56, No. 7, "Manufacturing technology for seamless pipes" (hereinafter referred to as prior literature),
There is a certain reduction rate that can suppress internal defects, and at least the characteristics of the material have to be grasped, so such a proposal is completely powerless. This is because the state of the billet shaft core, which is the source of internal flaws, is completely different depending on the steel type of the raw material or the manufacturing method because it undergoes severe processing such as Mannesmann drilling.

However, when the material is directly cast, especially when it is a high-alloy steel, the internal quality of the billet is significantly deteriorated, and the inner surface defects cannot be suppressed with the reduction ratio as shown in the prior art. The current situation. Further, even if an attempt is made to change the perforation conditions in order to obtain the reduction rate, the material will cause a bite failure this time, and eventually there is a problem that the inner surface defects cannot be suppressed.

[0012]

As described above, in order to obtain a product having a good inner surface quality by the conventional material manufacturing / pipe making process, it is necessary to pre-process the material, but the pre-processing is performed. Therefore, there is a demerit that a special capital investment is required and the cost will rise. On the other hand, if we try to manufacture a pipe while accepting the poor quality of the material, it will be accompanied by a relatively large-scale modification of the equipment, and it will be necessary to process each material individually. There was a problem that disadvantages occurred because it was not obtained. Therefore, it has been extremely difficult to prevent defects on the inner surface of the pipe even after pipes of difficult-to-machine materials, particularly as-cast low-alloy steel and high-alloy steel, have been produced through almost the same steps. Therefore, there is a long-felt need for a technology that solves the "demerit of cost increase" by using materials that are not subjected to pre-processing such as direct casting as much as possible, and without making internal flaws by drilling, without large capital investment. There is.

The present invention has been made in order to solve the above-mentioned problems, and the biting failure in the Mannesmann indentation perforation method using free-cutting steel whose internal quality easily deteriorates or high-alloy steel that is difficult to work as a material. It is an object of the present invention to provide a method for producing a seamless pipe which can effectively suppress the generation of inner surface defects without causing any defects.

[0014]

In view of the above circumstances, the present invention performs conventional manufacturing of difficult-to-machine materials and as-cast materials by extremely rational means that does not cause the disadvantage of increased cost burden. In addition, in order to suppress the inner surface defects of the pipe, we examined the maximum use of the existing equipment. As a result, the present invention that solves the above problems has been completed. Succession according to the present invention
The method for producing a seamless tube has a Cr content of 0.5 mass% or more.
As-cast slab made of high alloy steel with a round cross section
A joint that bites into an inclined rolling roll and pierces with a piercing plug.
In the method for producing a seamless pipe, the
The oblique rolling roll is rotated at a peripheral speed of less than 4.5 m / sec,
Pushing man while applying back pressure to the slab with a pusher
Nesmann drilling is performed, and the slab is stable on inclined rolling rolls.
Back pressure is applied to the slab with a pusher until it bites into
Perforation with a plug tip reduction defined by the following formula of 97% or more
A method for producing a seamless pipe, which comprises: M = d / D where M is the plug tip rolling reduction, and d is the plug tip position.
Roll interval, D is the diameter of the slab. The principle of the present invention will be described below.

Based on the findings of the above-mentioned prior documents, etc., in order to perforate and manufacture a material having poor workability without defects,
It has been suggested that light pressure perforation is necessary. In order to achieve this, it is necessary to advance the piercing plug toward the blank side to create a light reduction state. This seems to be contradictory, but it is possible to manufacture pipes without causing internal defects on any material if the Mannesmann effect, that is, the rotary forging effect is suppressed as much as possible while using the inclined piercing and rolling method. It means that

Therefore, as a result of intensive studies, the inventors of the present invention have found a technique that enables perforation with a much smaller rolling reduction than in the past and can suppress inner surface defects. That is, the rotation speed of the main roll used for perforation is less than 4.5 m / s, and in addition, the pusher conventionally used for perforation is used to the maximum, and the material is pushed to the roll by the pusher before biting failure occurs. This is a perforation method that suppresses inner surface flaws by pushing. Note that the roll rotation speed here is a speed immediately before the occurrence of a biting failure when punching is performed at a certain speed, that is, a speed at the biting limit, for example, a roll shape used for Mannesmann drilling is a barrel. In the case of a die, the maximum point, that is, the peripheral speed at the portion having the largest roll diameter is evaluated.

There is a certain limit under the normal piercing and rolling conditions for light reduction piercing, that is, the expansion of the plug tip reduction rate M defined by the following formula (1). However, d represents the roll interval at the plug tip position, and D represents the billet diameter. Also,
The rolling reduction N that the material receives from the roll and plug is expressed by the following formula (2).
Given in. For example, the plug tip rolling reduction M is 0.97.
In case of (97%), the material rolling reduction N is 0.03 (3
%).

M = d / D (1) N = 1-M (2) According to the above equation (1), increasing the roll interval at the plug tip is synonymous with increasing the plug tip rolling reduction. There are a perforating method for widening the roll interval and a perforating method for advancing the perforating plug in the billet direction in advance. In any case, the contact area from the billet contacting the roll to the contact with the plug is reduced, and it is difficult to obtain thrust from the roll to overcome the drag force of the plug. For this reason, after the raw material is bitten into the main roll, the raw material comes into contact with the perforated plug and receives a drag force in the rolling direction, so that the raw material does not advance.

On the other hand, the present inventors, based on a large amount of actual measurement data, when drilling a difficult-to-machine material such as a slab (for example, as-cast) of a high alloy steel or the like with deteriorated internal quality, 95% is required. It has been found that if a plug tip rolling reduction exceeding the above level can be secured, it is possible to suppress defects to a sufficient extent for industrial production, and further increase it to about 97% to eliminate internal defects almost completely. The result is shown in FIG. 7.

Further, the present inventors have made many studies on how efficiently the material can be drawn into the roll to give a thrust force that overcomes the drag force of the plug. As a result, it was concluded that it is advantageous to increase the friction coefficient between the roll and the material, and at the same time, to apply an external force so as to eliminate defective biting. These will be sequentially described below.

As a method for increasing the material pull-in force of the roll, unevenness has already been provided on the roll surface.
No. 251305 has been devised, but it cannot cope with the continuous change of surface irregularities due to the use of rolls,
There is also a problem that the product surface quality is deteriorated when the uneven shape is inappropriately large. On the other hand, regarding the friction coefficient between the roll and the material, for example, as disclosed in Japanese Patent Application Laid-Open No. 5-57307, a technique has been proposed in which a friction-increasing agent is supplied between the perforating roll and the material billet. However, this technique also reduces "slip" caused by deterioration of the roll surface, which is likely to occur in high-alloy steels, and compensates billet advancing efficiency with an increase in the number of holes. In addition, as clearly described in the examples of the publication, there is also an expression that among the application examples, those in which a flaw has occurred are material properties. In other words, internal flaws caused by the material are It cannot be suppressed. In any case, an effective rolling method capable of constantly maintaining a certain coefficient of friction and imparting thrust sufficient to overcome the drag of the piercing plug has not been confirmed.

Therefore, the present inventors have conducted a study focusing on the roll rotation speed. It has been said that there is a correlation between the frictional force and the roll rotation speed or rolling speed. However, this correlation is a matter confirmed by ordinary plate rolling using a roll whose surface is not processed at all and which is completely orthogonal to the traveling direction of the material. On the other hand, the piercing rolling is a completely different rolling in which the surface is sufficiently processed and the roll is placed on an axis having only a slight angle with the traveling direction of the material, as described above. Moreover, it hinders the progress of the material called the perforated plug. Furthermore, no correlation was found between the friction coefficient and the plug tip rolling reduction.

Therefore, the present inventors have conducted many experiments and have found that under certain conditions, there is a clear relationship between the roll rotation speed and the plug tip rolling reduction. In FIG. 7, the horizontal axis represents the roll rotation speed (m / sec), and the vertical axis represents the plug tip reduction rate at the above-mentioned bite limit, and the result of examination of the correlation between the roll rotation speed and the plug tip reduction rate is shown. FIG. As is apparent from this figure, perforation with a high plug tip reduction rate, that is, light reduction perforation is possible in a region where the roll rotation speed is low. From this, in the region where the roll rotation speed is less than 4.5 m / sec, the plug tip reduction ratio of 95% or more can be secured, and to some extent, like a material that has undergone a process such as rolling once before drilling with high alloy steel. A rolling condition of 95% for the tip of the plug tip, which is a rolling condition sufficient to suppress inner surface defects caused when drilling a material with secured internal quality, can be secured, and the quality of the pipe after actual drilling was investigated. It is clear that the suppression can be done on an industrial production basis.

By the way, the pusher is originally provided for the purpose of transporting the material to the inclined punching machine, and when the material billet is caught in the punching roll, the pusher automatically retracts,
The operator manually retracts during manual operation. In this case, the bite is automatically started when the rotating piercing roll and the material billet come into contact with each other, and the pusher has no special meaning other than the material conveyance. On the other hand, when the piercing plug is advanced to the material side and the material is pierced under a light pressure, the material driven by the frictional force between the roll and the material comes into contact with the piercing plug to produce the effect. As a result, the driving force applied to the material is exceeded, resulting in defective biting. In addition to the drag force caused by the perforated plug, insufficient factor of the bite may be insufficient transmission of driving force to the material due to insufficient frictional force between the roll and the material. However, in an attempt to solve this problem by increasing the frictional force, the perforated roll surface already has well-known unevenness called knurling or surface processing marks called knurling, so the roll due to aging changes. Except for surface variations, it was not possible to have a significant effect on bite. Therefore, when the problem of the drag force due to the piercing plug was thoroughly examined again, it was found that this biting failure occurred.
As shown in FIG. 1, the difference in the normal perforated state is only the balance between the driving force and the drag force due to the difference in the contact area between the roll and the raw material obtained by the deformation of the raw material during the normal perforation. The inventors paid attention to this point and applied a back pressure sufficient to overcome the drag of the piercing plug with a pusher so that defective biting did not occur, and a contact area in a pierceable state could be secured, and piercing proceeded normally. It has been found that it is possible to suppress perforation under light pressure, that is, to suppress inner surface flaws.

It has been recognized that the occurrence of inner surface defects can be suppressed to the level of conventional carbon steel even for direct cast materials of difficult-to-machine materials such as carbon steel containing a large amount of sulfur and high-alloy steel, whose internal quality is likely to deteriorate. It was

[0026]

Various preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

As shown in FIG. 1, the Mannesmann plug mill 1 is provided with a pair of upper and lower inclined rolling rolls 7, a pusher 5 provided on the upstream side of the inclined rolling rolls 7, and a downstream side of the inclined rolling rolls 7. And a mandrel device 9.

The inclined rolling roll 7 has a barrel-shaped peripheral surface, and has a maximum diameter in the central portion for biting and reducing the billet 2. The upper and lower rolls 7 are provided in a housing (not shown) so as to be able to move up and down via a hydraulic cylinder (not shown) and a roll chock. As shown in FIG. 2, the shaft 7a of the upper and lower rolls is laterally displaced by an angle θ with respect to the transport line of the billet 2.

The pusher 5 is rotatably connected to the hydraulic cylinder 3 via a clutch 4. The tip of the pusher 5 is provided so as to be able to contact the rear end of the billet 2, and the billet 2 is pushed and conveyed to the inclined rolling roll 7 while being supported and guided by a plurality of pinch rolls 6.

A mandrel device 9 is arranged downstream of the rolling roll 7. The mandrel device 9 includes a heavy-duty hydraulic cylinder and a piercing plug 8. The mandrel device 9 pushes the piercing plug 8 into the billet 2 that is caught between the upper and lower rolls 7.

Next, referring to FIG. 3 and FIG. 4, the case where a bite failure occurs, the case of normal piercing and rolling, and the case where the present invention is applied to the billet with the pusher while controlling the roll rotation speed, Description will be made by comparing with the case of applying pressure.

FIG. 3 (a) is a diagram schematically showing a state in which the billet 2 is not bited by the roll 7 and a so-called biting failure occurs. In such a defective biting state, the thrust load (plug load) and the roll load applied to the perforated plug 8 change as shown in FIG. That is, since the billet 2 is not caught in the roll 7, both of the plug load P1 and the roll load R1 are kept at a low level from the start time t1 to the continuation time t15 without increasing. As a result, the billet 2 is not perforated at all.

FIG. 3B is a diagram schematically showing a state in which the billet 2 is normally bitten into the roll 7 and then a normal perforation is performed. In the perforation under such a normal biting state, the thrust load (plug load) and the roll load applied to the plug 8 change as shown in FIG. 4 (b). That is, first, the billet 2 is bitten by the roll 7 at time t1, and the roll load R2 rapidly increases,
Next, the billet 2 is pushed into the perforated plug 8 and the plug load P2 suddenly increases. Peak time t5 to t10 of the roll load R2 and peak time t4 of the plug load P2
The roll load R2 during perforation is always higher than the plug load P2, although not necessarily equal to t9. When the piercing plug 8 finally penetrates the billet 2, both loads R2 and P2 decrease sharply and become zero at almost the same time t13.

FIG. 3C is a diagram schematically showing a state in which the back pressure is applied to the billet by the pusher while the roll rotation speed is controlled by applying the present invention. The difference between the normal perforation state and the defective biting state is the balance between the driving force and the drag force that the material receives from the perforation plug due to the difference in the contact area between the roll and the material, which is obtained by the material deformation during normal perforation. Only. As a result of earnest research efforts focusing on this point, the present inventors applied a back pressure to the material with a pusher to overcome the drag force of the perforated plug so as not to cause defective biting, and roll the material. We have found the conditions to make the contact area in a state where punching is possible and to shift to a normal punching state. That is, as shown in FIG. 7, when the back pressure is applied to the billet 2 by the pusher 5 and the roll rotation speed is less than 4.5 m / sec,
It was found that the plug tip rolling reduction M certainly exceeded 95%.

An outline of this mechanical mechanism is shown in FIG.
This will be described with reference to (c). Rolling roll 7 4.5m
When the billet 2 is pushed by the pusher 5 during the time t3 to t6 while rotating at a low speed of less than / sec, the roll load R3 and the plug load P3 gradually increase. In the initial biting, the load fluctuation is the same as when the biting is bad, but when the back pressure starts to be applied from the rear of the material by the pusher, the roll load R3
It is clear from the figure that both the plug load P3 and the plug load P3 have begun to increase, and have shifted to the normal perforation state midway. Since the pusher retracts at the stage when the roll load starts to increase, that is, when the contact area is started to be secured, no extra load is applied to the perforated plug. Therefore, even if a back pressure is applied from the rear side of the material, there is no fear of reduction in durability, which is a concern in high-alloy steel perforation. In addition, the troubles associated with the material biting between the rolls and the various guide shoes that would normally occur during perforation do not occur even if the pressing is performed because the perforation is performed with a light pressure. When the back pressure applied to the material overcomes the drag force of the perforated plug, the material is completely caught in the roll, and the roll load R3 and the plug load P3 both reach their maximum values. That is, the material comes into contact with the perforated plug after the mutual contact area between the roll and the material is sufficiently large. Peak time t7 to t12 of roll load R3 and peak time t of plug load P3
8 to t11 does not necessarily match, but both loads R3, P
3 has the maximum value. Roll load R3 in this area
Is large enough. Further, although the perforation time is extended in FIG. 4 (c), if the material is bitten into the roll and the normal perforation state is reached, if the roll speed is returned to the normal speed,
There is almost no loss of productivity and the occurrence of internal defects can be suppressed.

FIG. 9 is a characteristic showing the correlation between the plug tip reduction ratio and the inner lap flaw evaluation score when a large diameter seamless pipe is manufactured by using the method for manufacturing a seamless pipe according to the present invention and the conventional method. It is a figure. In the figure, a characteristic line A corresponds to a relational expression obtained by plotting various data by a method such as the least square method. As is clear from the figure, the inner lap flaw evaluation score becomes better as the plug tip reduction rate increases, and almost reaches the pass line (the inner lap flaw evaluation point is 50 points) in the region where the plug tip reduction rate exceeds 95%. It has been found. Particularly good results were obtained for difficult-to-work materials such as carbon steel (free-cutting steel) containing a large amount of sulfur.

FIG. 10 shows the correlation between the plug tip rolling reduction and the inner lap flaw evaluation score when a small-diameter and a large-diameter seamless pipe are manufactured using the seamless pipe manufacturing method according to the present invention and the conventional method. FIG. In the figure, a characteristic line B corresponds to a relational expression obtained by plotting various data by a method such as the least square method. As is clear from the figure, the inner wrap flaw evaluation score becomes better as the plug tip reduction rate increases, and in the region where the plug tip reduction rate exceeds 95%, the pass line (inner wrap flaw evaluation point is 50
Point) has been reached. Especially small diameter 13% Cr stainless steel tube (outer diameter 139.7 mm x thickness 7.72
In (mm), by securing 97% or more, excellent results were obtained with almost no internal defects. This gives 13% C
r It is possible to manufacture a seamless pipe of stainless steel from a directly cast material in the as-cast state.

Example 1 The present invention was studied with a hot model punch. As-cast 1 for material billet
% Cr steel, the material heating temperature is 1250 ° C., the roll-entry side angle is 2.5 degrees, the roll inclination angle is 9 degrees, the roll is barrel type, and the surface processing is 0.5 mm depth called normal knurling. A perforation experiment was carried out under the condition that roughening was performed to some extent. For internal defects, each level 10
The book was perforated, and the ratio of the length of the blank pipe in the portion where the flaw was generated to the total rolling length was evaluated as the defective rate. In Table 1, when the occurrence rate of inner surface defects is 15% or more, a cross mark is used, 5% or more and less than 15% is a triangular mark, 1% or more and 5% or more.
Less than 1% is indicated by a single circle mark, and less than 1% is indicated by a double circle mark. As is clear from Table 1, in the comparative example in which the punching was performed under the condition where the pusher was not used, the reduction rate of the plug tip could not be made high, and there was a tendency that the inner surface flaw increased with the increase of the roll rotation speed. At 5.8 m / sec, the defect rate was 25.5% and the defect was not suppressed (Sample No. 2). However, when pushing with a pusher,
Further, in an example in which low-speed perforation with a roll rotation speed of less than 4.5 m / sec is applied, the plug tip rolling reduction is 97%.
Can be obtained (sample number 8). By the way, when the roll rotation speed was set to 4 m / sec, it was possible to punch without causing any internal flaw (Sample No. 9).
Further, increasing the plug tip reduction rate M, that is, light reduction drilling, means advancing the plug position to the material entry side, which is equivalent to expanding the roll interval at the plug tip. Therefore, the material billet diameter is The fact that it does not change makes the roll interval wide, which is consistent with the fact that a light reduction state is created. Therefore, this example shows that the above result, that is, the application of the present invention leads to the reduction of inner surface defects.

(Example 2) Under the same perforation conditions, 13% as cast, which is inferior to Example 1 in hot workability as a raw material
Table 2 shows the results of examining the occurrence of internal flaws using a Cr-3% Mo-5% Ni steel material. As is clear from Table 2, in the second embodiment, the same as in the first embodiment, 6.3.
In the comparative example (Sample No. 10) in which pushing by the pusher was not used at the roll rotation speed of m / sec, the defective rate was very high at 58%, and industrial production was not very feasible. However, when the method of the present invention is applied, the roll rotation speed is 5.8.
Applying a pusher force of 96 m / sec, it is possible to secure a 96% plug tip reduction rate, with a defect rate of 13.5% (Sample No. 15), and a slower 4.45 m / sec 97% plug tip. A reduction rate can be secured, and 4.
Pipes can be made with a defect rate of 5% (Sample No. 1
7) Finally, it was found that a pipe can be manufactured in a state of being almost intact (Sample No. 19) by applying a combination of low speed perforation of 3.4 m / sec and indentation. As a result, we have obtained the prospect that it will be possible to manufacture the actual machine with almost no maintenance. It was confirmed that the cost of material pre-processing can be reduced as it is according to the present invention.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

EFFECTS OF THE INVENTION According to the present invention, when piercing and rolling a difficult-to-cast material which is conventionally unsuitable as a seamless pipe material due to poor hot workability due to the presence of zaku and the like in the center of the material. Moreover, it is possible to optimize the drilling conditions without making a large change in the equipment. As a result, it is possible to manufacture a pipe with a high plug tip reduction rate of 97% without causing defective biting, suppress the occurrence of inner surface defects, and manufacture a high-value-added high alloy steel seamless pipe at low cost. Therefore, productivity can be improved.

Particularly in the case of a small diameter pipe, even a difficult-to-work material such as 13% Cr stainless steel or free-cutting steel produces a seamless pipe from a directly cast material in the as-cast state with almost no inner surface defects. It is possible.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a seamless pipe manufacturing apparatus using a Mannesmann indentation perforation method.

FIG. 2 is a plan view showing an inclined rolling roll.

3A is a schematic cross-sectional view showing a state of defective biting, FIG. 3B is a schematic cross-sectional view showing a normal perforation state, and FIG.
[Fig. 3] is a schematic sectional view showing a state in which perforation is continued by being pushed in by a pusher.

FIG. 4 (a) is a load fluctuation characteristic diagram showing changes over time in perforation plug load and roll load at the time of defective biting, and FIG. 4 (b).
Is a load fluctuation characteristic diagram showing the changes over time of the perforation plug load and roll load during normal drilling, and (c) is a load fluctuation characteristic line showing the changes over time of the perforation plug load and roll load when the punching is continued by pushing the pusher. Fig.

FIG. 5 is a viscosity characteristic diagram of molten steel showing the relationship between the Cr content and the viscosity of molten steel.

FIG. 6 is a viscosity characteristic diagram of molten steel showing the relationship between the Cr content and the viscosity of molten steel.

FIG. 7 is a characteristic diagram showing a correlation between a roll rotation speed and a plug tip rolling reduction when various seamless pipes are manufactured by using the seamless pipe manufacturing method according to the present invention and a conventional method.

FIG. 8 is a schematic cross-sectional view showing, in an enlarged manner, the rolling roll, the piercing plug, and the work at the time of piercing in order to explain the relationship between the geometrical relationship of the rolling roll, the piercing plug, and the work and the plug tip reduction ratio.

FIG. 9 is a characteristic diagram showing the correlation between the plug tip reduction ratio and the inner lap flaw evaluation score when a large-diameter seamless pipe is manufactured using the seamless pipe manufacturing method according to the present invention and the conventional method.

FIG. 10 shows the correlation between the plug tip rolling reduction and the inner lap flaw evaluation score when a small-diameter and a large-diameter seamless pipe are manufactured using the seamless pipe manufacturing method according to the present invention and the conventional method, respectively. Characteristic diagram.

[Explanation of symbols]

2 ... billet, 3 ... hydraulic cylinder, 4 ... clutch, 5 ... Pusher, 6 ... pinch roll, 7 ... inclined rolling roll, 8 ... perforated plug, 9 ... Mandrel device.

Continuation of the front page (56) Reference JP-A-10-180312 (JP, A) JP-A-59-50906 (JP, A) JP-A-63-220907 (JP, A) JP-A-10-175049 (JP , A) JP 62-282713 (JP, A) JP 6-218406 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B21B 19/04

Claims (2)

(57) [Claims] 1. A high alloy having a Cr content of 0.5 mass% or more.
3. In the method for producing a seamless pipe in which an as- cast slab made of steel and having a round cross section is bitten into an inclined rolling roll and perforated with a piercing plug, the inclined rolling roll is punched every second when piercing the slab . 5m
Back pressure on the slab with a pusher
While pushing the punching mannesmann, back pressure is applied to the slab by the pusher until the slab is stably bitten into the inclined rolling roll, and the plug tip defined by the following formula
A method for producing a seamless pipe, characterized in that perforation is performed at a reduction rate of 97% or more . M = d / D where M is the plug tip rolling reduction, and d is the plug tip position.
Roll interval, D is the diameter of the slab. 2. The method for producing a seamless pipe according to claim 1, wherein the speed of the pusher is set within a range not exceeding the speed of the slab in the rolling direction.